1. Field of the Invention
The present invention generally relates to an. image forming apparatus for electrophotographically forming an image and, more particularly, to an image forming apparatus provided with a plurality of developing devices each containing toner of a different color.
2. Description of the Related Art
An image forming apparatus, in which a plurality of toner images of different colors are transferred all together onto a transfer material, is known. For example, when toners of two different colors are used in such an image forming apparatus, a first electrostatic latent image formed on the surface of a photosensitive body is initially developed by the first toner contained in a first developing device. A second electrostatic latent image is then developed in an overlapping manner by the second toner of the same polarity as the first toner in a second developing device. Thereafter, toner images formed by the first and second toners are both transferred onto a transfer material. According to this type of apparatus, since it is so arranged that the first toner image passes a developing area of the second developing device, part of the toner of the first image is disadvantageously scraped off by the action of a magnetic brush of a second developer material including the second toner and eventually mixed into the second developing device. As a result, image quality is deteriorated due to the mixture of first and second toners.
In order to avoid the above-described disadvantage, such a proposal is heretofore made that the first and second toners consist of non-magnetic toner and magnetic toner, respectively. Alternatively, both the first and the second toners are rendered magnetic, but the second toner consists of toner of higher magnetism than the first toner. The first toner can accordingly be selectively collected because of the difference in developing threshold value between the first and second toners.
However, according to the above-proposed method, not only the first toner is collected from among the second developer material, but the second toner is adhered to a collecter by the mechanical force such as the adhesive force thereof or the like to the collecter. In this case, there arises no problem if the amount of the second toner adhered to the collecter is considerably small. In the above-described conventional method, however, the second toner is actually collected up to 10-20% of the whole collected toner. Therefore, a good deal of the second toner is wasted without being served for the development.
In the meantime, Japanese Patent Laid-open Publication (unexamined) No. 58-137846 proposes a method for separating and collecting the first toner mixed into the second developing device. The order of frictional electrification is suitably set for the first toner, the second toner and carrier according to this method, so that the first toner mixed into the second developer material may be separated from the carrier by reversing the polarity thereof to a polarity opposite to the second toner. The separated first toner is scattered out of the developing device or collected at a non-image area of the photosensitive body or by a collecting roller.
However, the above method also has the following disadvantages. That is, the control in the order of electrification is required, which narrows in many cases the allowance of the composition of toner. Since the electrification is closely related to environments, particularly humidity, the image forming apparatus should be installed at a proper place so that the resulting images may not be subject to variation. Moreover, the scattered first toner may spoil the inside of the image forming apparatus.